1) Technical Field of the Invention
The present invention relates to a semiconductor device and a manufacturing process thereof, and in particular to an IGBT (Insulated Gate Bipolar Transistor) and a manufacturing process thereof.
2) Description of Related Arts
Various power conversion circuit devices have been proposed for driving motors of recent industrial equipments such as elevators and pumps. While the power conversion circuit devices may be categorized into direct and indirect ones, the indirect (inverter-control type) power conversion circuit device has commonly been used in the art so far. In the meantime, the direct power circuit such as an AC matrix converter has many advantages superior to the inverter control type power conversion circuit, including downsizing and extended lifetime of the power conversion circuit device due to elimination of the electrolytic capacitor.
The AC matrix converter includes, in general, a plurality of bidirectional switches, each of which may be structured by a pair of IGBTs reversely connected in parallel. Thus, the IGBT used for the AC matrix converter blocks voltage to have high withstanding voltage both in forward and reverse directions during turning-off. In the context of the present application, the IGBT blocking voltage not only in a forward direction but also reverse direction is referred to as a “reverse blocking IGBT”.
In the bidirectional switch fashioned with a pair of the reverse blocking IGBTs, one of the reverse blocking IGBT controls the driving current therethrough, and the other one allows the recovery current from the load by turning on after the former turns off. Thus, the latter IGBT has a function serving as a free wheel diode in the inverter-control type power conversion circuit device, running the recovery current from the load in the direction reverse to the driving current. When running the recovery current, the latter IGBT generates a considerable amount of recovery loss since the p+-type collector region (collector layer) thereof has a substantial thickness and a high impurity concentration.
The Japanese Patent Publication Application, JP 2002-319676, A, for example, discloses the technique to thermally anneal the p+-type collector region for activating the impurity ions therein and obtaining desired peak impurity concentration, thereby reducing the recovery loss. However, this approach reduces the recovery loss, yet deteriorates the withstanding voltage (breakdown voltage) in an intolerable manner. The above-referenced patent application suggests that the annealing temperature is preferably less than 500 degrees centigrade, because the higher temperature raises the ohmic contact resistance between the emitter region and the emitter electrode. Such rather low annealing temperature is insufficient to activate the impurity ions in the collector region, so that lattice defects due to the doped impurity remain in the collector region. Thus, when applied with reverse voltage, the depletion region likely extends over the collector region with the lattice defects, a leak current likely runs through the lattice defects so that the withstanding voltage is reduced.